ConstNo StopTimeEvents are given for this stop. The main intention for this value is to give time predictions only for part of a trip, i.e., if the last update for a trip has a NO_DATA specifier, then StopTimeEvents for the rest of the stops in the trip are considered to be unspecified as well. Neither arrival nor departure should be supplied.
The vehicle is proceeding in accordance with its static schedule of stops, although not necessarily according to the times of the schedule. At least one of arrival and departure must be provided. If the schedule for this stop contains both arrival and departure times then so must this update. Frequency-based trips (GTFS frequencies.txt with exact_times = 0) should not have a SCHEDULED value and should use UNSCHEDULED instead.
The stop is skipped, i.e., the vehicle will not stop at this stop. Arrival and departure are optional.
The vehicle is operating a trip defined in GTFS frequencies.txt with exact_times = 0. This value should not be used for trips that are not defined in GTFS frequencies.txt, or trips in GTFS frequencies.txt with exact_times = 1. Trips containing StopTimeUpdates with ScheduleRelationship=UNSCHEDULED must also set TripDescriptor.ScheduleRelationship=UNSCHEDULED. NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
The relation between the StopTimeEvents and the static schedule.